Currently, various display devices are actively being researched and developed, particularly those based on electroluminescence (EL) from organic materials. Contrary to photoluminescence (i.e., light emission from an active material due to optical absorption and relaxation by radioactive decay of an excited state), EL refers to a non-thermal generation of light resulting from applying an electric field to a substrate. In the case of EL, excitation is accomplished by recombining the charge carriers of opposite signs (electrons and holes) injected into an organic semiconductor in the presence of an external circuit.
A simple prototype of an organic light-emitting diode (OLED), i.e., a single layer OLED, is typically composed of a thin film made from an active organic material, which is sandwiched between two electrodes. One electrode needs to be semitransparent in order to observe the light emission from the organic layer. Typically, an indium tin oxide (ITO)-coated glass substrate is used as an anode.
Many organic materials exhibit fluorescence, i.e., luminescence from a symmetry-allowed process, from singlet excitons, which may be efficient since this process occurs between states of the same symmetry. On the contrary, if the symmetry of an exciton is different from the one of the ground state, then the radioactive relaxation of the exciton is disallowed and the luminescence will be slow and inefficient. Since the ground state is usually anti-symmetric, the decay from a triplet breaks the symmetry. Thus, the process is disallowed and the efficiency of EL is very low. Therefore, the energy contained in the triplet states is mostly wasted.
Luminescence from a symmetry-disallowed process is known as phosphorescence. Typically, phosphorescence may last up to several seconds after excitation due to the low probability of the transition, which is different from fluorescence that originates in a rapid decay. However, only a few organic materials have been identified that show efficient room temperature phosphorescence from triplets.
If phosphorescent materials are successfully utilized, they hold enormous promises and benefits for organic electroluminescent devices. For example, the advantage of utilizing phosphorescent materials is that all excitons (formed by combining holes and electrons in an EL), which are, in part, triplet-based in phosphorescent devices, may participate in the energy transfer and luminescence. This can be achieved by phosphorescence emission itself or, alternatively, it can be accomplished by using phosphorescent materials to improve the efficiency of the fluorescence process as a phosphorescent host or a dopant in a fluorescent guest, with phosphorescence from a triplet state of the host enabling energy transfer from a triplet state of the host to a singlet state of the guest.
Iridium(III) complexes have recently attracted a lot of interest as potential triplet emitters in electronic devices and in biological applications as luminescent and electrochemiluminescent materials. Colors ranging from bluish green to red are generated by varying the ligands in the iridium complexes. However, iridium complexes having only one iridium atom have emitted light at a very narrow spectral region and, thus, are not suitable for white light emission, e.g., for replacing incandescent bulbs. If each light-emitting diode (LED) emits only at a narrow spectral region, a white light display would require multiple LEDs, and the multiple LEDs would then be incorporated into complicated and expensive LED modules to obtain the required broad band illumination necessary for providing white light. In this regard, there have been several studies on the development of iridium complexes having at least two iridium atoms.
Plummer et al., “Mono- and Di-nuclear Iridium(III) Complexes: Synthesis and Photophysics,” Dalton Trans., 2080-2084 (2003) discloses heteroleptic mono- and di-nuclear iridium(III) complexes containing two ortho-metalating ligands, 2-phenylpyridine with a bipyridine derivative. The iridium(III) complexes emit light in the range from a green region to a red region, as represented by the following formula:

Tsuboyama et al., “A Novel Dinuclear Cyclometalated Iridium Complex Bridged with 1,4-bis[pyridine-2-yl]benzene: Its Structure and Photophysical Properties,” Dalton Trans., 1115-1116 (2004) discloses a dinuclear iridium complex, which exhibits intense red phosphorescence in solutions, as represented by the following formula:

L. Yang., “Novel HEXOL-Type Cyclometallated Iridium(III) Complexes: Stereoselective Synthesis and Structure Elucidation,” Chem. Commun., 4155-4157 (2005) discloses the preparation of two diastereoisomers of tetranuclear cyclometallated iridium complexes, either having an inner core of HEXOL-type [Ir(IrCl2)3]6+ unit and a surface of six chiral, didentate, cyclometallated ligands from an enantiopure pinenopyridine derivative.
PCT International Publication No. WO 2004/043974 relates to a process for producing a trivalent hexadentate ortho-metallated iridium complex characterized in a monovalent iridium dinuclear complex having halogens as bridging ligands.
However, the above light-emitting materials containing iridium complexes disclosed in the art do not exhibit sufficiently high efficiency as well as broad emission to obtain white light. It would thus be desirable to develop light-emitting materials capable of effectively emitting light of a broad spectral range, especially in the orange, green, and blue regions.